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19 March 2017

How to make a King-B Piezo Booster Pedal

Can you spot the subtle difference in title from the last post I made on this blog? My last post talks at length about my success in making an impedance handling buffer pedal specifically targeted for use with Piezo pickups. Today's post talks about my trials and tribulations in my quest to make a Booster pedal (again specifically targeted at Piezo pickups). This new project  really pushed me to my limits as I hit problem after problem. I persevered and the results are well worth the effort! Pretty much everything that could go wrong did and a few more things besides. I'll try and remember it all for you below... Who ever learnt anything from getting it right all of the time anyway?

Before we get stuck in, let's take a quick look at my latest lino-cut. This is one I'm calling "King-Z" and is inspired by my King-Z circuit and how I laid out the actual board for that project. I used a little artistic license here and there, but I'm pleased with the end result. The world needs more circuit-inspired art! Go make it! 

This project is a step up from the last one, but thankfully it neatly builds on a lot of the ideas and theory we used to get the 2N5457 JFET working. I will call out right now that the circuit is inspired by the original MXR Microamp. This pedal has given rise to a multitude of clones. The builders of the clones don't often give credit for their inspiration, but I will. If I can remember the differences in my circuit and the original, I'll call them out below. The main difference you'll discover is that I have specifically biased my circuit for Piezo pickups... but you already know this. I think that the MXR Microamp originally started life as the input stage to a number of MXR guitar effects pedals before the makers realised that it might just work well on its own. And they were right. Electric guitarists use MXR Microamps to boost signal in chained effects or to provide a little zing to a solo. I'm just looking to boost signal. The rest is a bonus ;-)

Here's the list of parts you will need to build the booster:

JFET-input operational amplifier - TL061
LED - 2.8V
R1 = 1k
R2 = 100k
R3 = 100k
R4 = 2k2
R5 = 10M
R6 = 10k
R7 = 22M
R8 = 1k
R9 = 10k
R10 = 680
R11 = 56k
R12 = 10k
R13 = 470
C1 = 34nf
C2 = 220uf
C3 = 0.1uf (“104”)
C4 = 4.7uf
C5 = 6.8uf
C6 = 10uf
C7 = 47pf

First let's tackle the power supply. This was a bloody nightmare to get right. What I built is NOT what I intended to build, but I'm documenting what I ended up with (even if I don't fully understand it).

My first design and prototype used an LM317T voltage regulator which was supplying a sweet 6 volts to the circuit. But just as I set out to build the actual circuit, I started to have second thoughts as I wondered whether I should add a heatsink for the LM317T. Even though I did the maths and figured out that I wouldn't need a heatsink, I ultimately dropped the LM317T. And here in lies my biggest mistake... I changed the design mid-flight! Boy, did I regret that later on.

R2 and R3 are simply there to divide the voltage (a technique I explained in the Buffer post). The TL061 requires power to be supplied to Vcc+ to get it to work AND to the input audio signal (via R2 and R5 in my circuit). The datasheet instructs you to have less voltage on the audio input than the Vcc+ and that the audio input voltage must not exceed 4.5v. It seems that everybody simply halves the 9v Vcc+ voltage, so this is what I did too.

I can't explain R1. It is a bit of residual DNA from multiple hack and slashes I undertook while troubleshooting problems. In my mind it isn't adding anything at all to this circuit, but it's there in my working pedal and so I'm showing it to you. If you build this circuit, experiment with taking it out and let me know what you discover. I think this might have been me trying to reduce the voltage even further across the circuit, but then I hit problems.

These two same capacitors are in the circuit for the Buffer Pedal and they serve the same purpose here. Go check that article out for more detail. I didn't have them with my original LM317T design, but they are definitely required for the Booster circuit as it stands now; They remove a mains hum that my circuit was suffering from in testing. And a pretty good job they do too!

Here I'm highlighting the DC isolation capacitors. AGAIN, this is exactly what we talked about with the Buffer Pedal. Isn't it nice when that happens! The values I'm using here are tailored for the TL061.

The combination of C5 & C6 are simply me getting around the problem where I didn't have the right capacitor size to allow me to use just the one. From my scribbled notes I think that the original MXR calls for a single 15uf capacitor here.

If there is any magic at all in this circuit then this must surely be it!

The highlighted part is what is required to bias the TL061. The values of components I have here are what makes this circuit suited for my specific needs. C7 and R11 are pretty standard, the rest, not so.

R9 is a potentiometer. The original MXR used an anti-log pot here, but I don't have one of those and I figured out that I could solve the problem with a standard 10k pot if I wired it up right. R10 acts as an optional feature if you want to be able to tone things back a little bit. I have it on a switch that either engages it or doesn't by short-circuiting the resistor.

It's a little tricky to get your head around what's happening here as it is back-to-front thinking. Zero resistance in R9 and R10 provides a loud overdriven sound (the "maximum overdrive"), and as you increase the resistance, the sounds quietens. There comes a point where adding resistance makes no discernible difference to the overall volume. For my circuit it's about 10k ohms.

The lowest logical setting for my Booster amp is being achieved by providing a bias of 10k + 680. In the "Low Boost" mode (with the 680 resistor switched in), the lowest resistance we can get is 680 ohms (that 680 ohms makes a difference... trust me). In the "High Boost" mode (with the 680 switched out), we can get all the way down to an ear-splitting zero ohms.

Here's how you wire up the pot to allow the sweep to increase volume as you turn clockwise and decrease when you turn anti-clockwise. I'll leave the switch wiring up to you should you want to fit R10 ;-)

Finally, I'm highlighting the input and output stages.

I explained a lot of this in the Buffer post, so this time, I'll just call out a couple of things specific to this circuit:

R5 and R7 are as per the MXR. They're far bigger than I would like, but it works so don't knock it.

R12 and R13 are NOT as per the MXR. This isn't a typo. You'll see that I swapped the two resistors around as I tuned this for the best audio I could get.

Look at this bloody rat's nest!

I managed to get it all into a box, but it's a bit of a tight squeeze!

I blew everything I could in this project. I think I'm right in saying that I destroyed 3 x LEDs, 2 x TL061s, one pot, a multimeter and my soldering iron! The only thing that I didn't fuse was the house mains! Ha ha. What a liability!

The biggest debugging challenge I faced was the old favourite "no sound" problem when I'd got it all together and soldered in place. I did all the usual debugging tricks, even at times resorting to replacing components one by one until I hit on the problematic areas.

It was only my poor soldering that saved me in one instance when pressing a terminal block brought the circuit into life. This was no bad joint though it turned out, but instead a short-circuit I hadn't managed to spot as I checked for continuity around the board. What a nightmare! Things were touching that shouldn't have been. Gah!

I suppose you're wondering how this sounds? Let me show you...

If I've done this right, you can see a couple of examples of the pedal in action in this video. First up is my homemade Sharkfin travel uke. It works well, but I think that the Buffer pedal might have the edge for this particular piezo. I don't play anything too exciting here, but you do get a feel for the range of the pedal. I can't see me using the "squeal like a pig" mode, but never say never ;-)

 Next up is my trusty guitar. I've never used this through an amp because it sounds so bad amped up. Well, listen to it through the Booster pedal... amazing! Unfortunately the recording doesn't quite do the sound justice. I found that the pedal worked best on the guitar with a low to mid boost. Adding reverb made it peachy ;-)

I wonder what a true electric might sound like... an experiment for another day.

Here's a photo of the boxed pedal sat next to its brother. I haven't tried chaining them... another experiment for another day.

Finally, I leave you with a recommendation. See this "Classic Ragtime Guitar Solos" by Stefan Grossman. I picked it up a couple of weekends ago. It is brilliant. I buy books like this all of the time from random charity shops. Rarely do I actually learn anything from them. Song one in this book is an old Tom Turpin song called The Bowery Bucks arranged for guitar by Dick Fegy. The original was played on piano. I see that a scan of the sheet music is available from the Library of Congress if you can read music.

I've been learning the tune in-between everything else going on and I love it. It's the tune I murder in the video above. One day I will play the whole thing for you fumble-free... perhaps ;-)

That's your lot!

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